An Organic Light Emitting Diode (OLED) display has got a lot of attention at present because it has advantages of a low power consumption, a high brightness, a low cost, a wide angle of view, a rapid response speed, etc, which has been applied widely in a field of the organic light emitting technique.
The OLED display drives a light emitting device to emit light is in a manner of direct-current driving. A capacitor is charged before the light emitting device is driven to emit light, and then a data signal is written into the capacitor; the capacitor is discharged when the light emitting device is driven to emit light for display, a discharging voltage of the capacitor decides the driving current flowing through a driving transistor, and the driving current from the driving transistor drives the light emitting device to emit light.
Generally, in a pixel circuit without any compensation sub-circuit, the driving current id driving the light emitting device to emit light is in proportion to (Vdd−Vss−VData−Vth)2. Wherein the Vdd is a high level reference voltage, the Vss is a low level reference voltage, the VData is a data signal voltage provided by a data line, and the Vth is a threshold voltage of the driving transistor.
A pixel structure in the current OLED display has following problems.
Firstly, the plurality of driving transistors for driving the respective light emitting devices to emit light and display on a backboard are non-uniform in the structure because of their manufacture processes and are also non-uniform in their electrical performances and stabilizations, which cause the threshold voltages Vth of the respective driving transistors shift with respect to a preset value Vth0. Secondly, the stabilization of the driving transistor would decrease in a case that the driving transistor has been turned on in the manner of direct-current driving for a long time, such that its threshold voltage Vth would also change.
Additionally, with the development of maximization of the OLED size, loads on a signal line become great accordingly and the number of the pixel circuits increases. A same power supply (a power supply providing the high level reference voltage Vdd) provides power supplies for different pixel circuits, and thus the voltage Vdd decreases evidently with the increasing of wirings such that the voltages Vdd actually provided to the respective pixel circuits are different from each other. When a same data signal VData is applied to each pixel circuit, the currents flowing through each OLED in a display area on the backboard are not identical due to the different values of Vdd, which cause the currents on the backboard to be non-uniform, such that a brightness in an image is non-uniform.